The fole of calcium in the regulation of ion channels, downstream signaling, and gene expression was investigated. We have been characterizing an endogenously expressed and functional muscarinic receptor on the surface of the A2058 human melanoma cell line. This receptor, in low abundance, was difficult to characterize using selective receptor inhibitors. The biochemical characterization of this receptor suggested that this receptor was novel in its signaling pattern or representative of a hybrid of the odd- and even-numbered muscarinic receptors. PCR studies have indicated that the A2058 cells express only one type of the muscarinic receptor on their surface. The message for this receptor cross-hybridizes with the human m5 mAChR and sequence studied to date is identical. Further sequencing is ongoing. This is the first demonstration of an endogenous, functional m5 receptor outside of the central nervous system.